The present invention relates generally to an improved thermally conductive and electrically insulative laminate for use as a mounting pad and electrical chassis barrier in combination with solid-state electronic devices, particularly wherein it is desired that an electrically insulative layer be interposed between the thermally conductive base of the solid-state device and the chassis or other ultimate heat sink. The properties required are that the laminates be both thermally conductive, electrically insulative, and conformable, a combination of properties not readily found in nature.
Materials, including elements, compounds, and compositions of matter rarely possess the combined properties of being both thermally conductive and electrically insulative. Since the number of materials possessing such a combination of properties is relatively limited, one must seek compromises in other physical and electrical properties in order to find a useful material. Also, one technique for decreasing the thermal impedance in an electrically insulative material is to provide a material with an extremely thin cross-sectional thickness. However, as the cross-sectional thickness decreases, the risk of rupture, cracking, or fracture of the material increases, thereby increasing the risk for electrical failures.
Further desirable properties or characteristics include toughness, and mechanical durability, these properties rendering the barrier member resistant to cutting, ripping, cracking, or puncturing. In addition, it is desirable that the barrier member be at least modestly pliable so as to increase the area of surface contact in order to maximize the heat transfer. The mechanical properties are desirable in order to provide an electrical chassis barrier member which is sufficiently tough and durable to withstand the forces of oven-torqued mounting screws, and furthermore reduce the occurrences of burr cut-through or cracking, which are frequent occurrences in production operations.
With respect to other physical-thermal properties which are desirable for use in combination with high power type solid-state devices, and in addition to being thermally conductive, it is desirable that the material possess a high temperature capability so that exposure to molten solder may be tolerated. Accordingly, wave-soldering techniques may be employed with the product of the present invention, and high temperature problems such as high temperature creep will not be likely to occur. Furthermore, because of the demands of production processes, both solvent and chemical resistent properties are desirable.
In the past, it has been suggested that a polyimide (amide) film be utilized for use in combination with power type semiconductor devices, such as transistors and the like. Additionally, wax coatings or the like have been suggested for use with such films, however it has been found that such coatings are undesirable, particularly from the standpoint of high-temperature deterioration which may result over periods of extended usage in combination with this type of device.